1. Field of the Invention
The present invention relates to an AM (Amplitude Modulation) detecting apparatus that adjusts the phase of its voltage-controlled-oscillation signal (VCO signal) in such a manner that it maintains the phase difference between the amplitude modulation signal (AM signal) and VCO signal at a specified value.
2. Description of Related Art
A conventional AM detecting apparatus adjusts the phase of its VCO signal in such a manner that it maintains the phase difference between the AM signal and VCO signal at a specified difference, and outputs the VCO signal after the phase adjustment to a coherent detecting circuit.
In overmodulation in which the modulation factor of a video signal exceeds 100%, however, the phase of the AM signal is inverted, making it impossible to carryout the coherent detection of the video signal in the AM signal.
To curb the adverse effect of the phase inversion, the conventional AM detecting apparatus attenuates the AM signal in the overmodulation, and supplies the attenuated signal to the coherent detecting circuit (see, Relevant Reference 1).
Relevant Reference 1: Japanese patent application laid-open No. 2000-31745 (paragraphs [0027] to [0034], and FIG. 1), which is incorporated herein by reference.
With the foregoing configuration, the conventional AM detecting apparatus can mitigate the adverse effect caused by the phase inversion of the AM signal in the overmodulation. However, it has a problem of being unable to carry out the coherent detection of the video signal in the AM signal in the overmodulation.